Insulated electrical conductor



June 23, 1925. 1,542,863

E. E. F. CREIGHTON INSULATED ELECTRICAL CONDUCTOR Filed Jan. 10, 1918Inventor Elmer E. Fcreigbton Hi 5 frtcorneg.

to suc r to layer.

Patented June 23, 1925. v

UNITED STATES 1,542,863 PATENT OFFICE.

ELMER E. F. CBEIGHTON, OF SCHENECTADY, NEW YORK, ASSIGNOR TO GENERALELECTRIC COMPANY, A CORPORATION OFNEW YORK.

INSULATED ELECTRICAL CONDUCTOR.

Application filed January 10, 1918. Serial No. 211,111.

To all whom it may concern:

Be it known that I, ELMER E. F. CREIGH- TON, a citizen of the'UnitedStates, residing at Schenectady, in the county of Schenectady, State ofNew York, have invented certain new and useful Improvements in InsulatedElectrical Conductors, of which the following is a specification.

My present invention relates to insulated electrical conductors andmethods of pre parin insulating material and applying it conductors. Itis common practice in manufacturing conductors with a covering offibrous insulating material, such as paper, to first wra the insulatingmaterial around the con uctor in successive layers until a covering ofthe desired thickness has been obtamed. The conductor thus covered isthen wound u on a reel and placed in a tank where it 1s subjected toheat treatment in a artial vacuum in order to remove moisture rom theinsulating material. There is, however, a limit to the amount ofmoisture which can be removed in this way from paper without destroyinga large part of .its mechanical strength and thus rendering it unfit forthe urpose for which it is used. As a result of t is treatment, also,the outer layers of aper on the conductor have more of the moistureremoved therefrom than the inner layers so unless the treatment iscontinued for a much longer time than would be commercially practicable,there must necessarily be a moisture gradient from layer After treatmentin this manner there is still suflicient moisture left in the insulationso that it has an easily measurable ohmic resistance and also a negativetemperature characteristic. This negative temperature characteristic ismanifested 1n two ways, first by a decrease in resistance with anincrease in temperature and second, by a decrease in puncture voltagewith an increase in temperature. Furthermore, such insulation has ahigher hysteresis loss with high frequency current than is desirable.

The object of my invention is to provide a method of removing more ofthe moisture from the insulation than has been possible with the methodsheretofore employed without appreciably lowering the mechanical strengthof the insulation. A further object of my invention is to provide aninsulated conductor which has an improved temperature characteristic inso far as the puncture voltage is concerned and the insulation of whichhas a higher resistance and lower hysteresis losses than insulation ofthe sam class previously used.

The usual oil impregnated paper has a decided negative characteristicas.already mentioned. By special treatment this negative characteristicdecreases and in some cases may even become decidedly positive; that isto say, the puncture voltage increases with the temperature within safelimits of temperature for the fibrous material.

In carrying my invention into effect I treat the insulating material insuch a way that a portion of its natural moisture or its moisture ofconstitution is replaced by an insulating fluid such as oil. By the termmoisture of constitution I refer to that moisture which forms a part ofthe cell structure and the removal of which under ordinary conditions,will. destroy the continuity of the cell structure or the cohesionbetween cells and thereby decrease the mechanical strength of thematerial. This object may be accomplished in a variety of Ways. Thematerial may first be dried by ordinary heat and vacuum treatment to ashigh a degree as possible without greatly weakening it, and thenimpregnated with oil and subjected to further heat and vacuum treatment.The removal of moisture at this stage may be facilitated by causing ablast of well dried warm air at a few centimeters pressure to pass overthe material. The material which is being treated should be sov arrangedthat as large a surface as possible will be exposed in order that themoisture will have a minimum distance to travel through the materialbefore being liberated. I have found that by this treatment a muchlarger amount of moisture may be removed from the material than with thepreviously used methods before its mechanical strength is greatlyreduced.

Similar results may be obtained by boiling the material in oil. I havefound that paper eight mils in thickness, which has been treated by theusual methods, if immersed in oil in separate layers at a temperature of105 to 110 C. for forty-eight hours will have its puncture voltage1ncreased to 16 kilovolts from a value which may vary from 5 kilovoltsto 12 kilovolts. Before the treatment in oil the puncture voltage of thesame paper decreased with increase of temperature from 22 C. to 107 C.about 40%, while after treatment the puncture voltage over the sametemperature range increased from 12 kilovolts to 16 kilovolts. When thismethod is employed the paper or other material should be so arrangedthat every layer Will be in direct contact with the oil in order thatthe moisture will not have to pass through more than one layer before itis liberated. By this same treatment the insulation resistance of thepaper may be measurably increased. The hysteresis losses may also bedecreased by a large amount and the danger of puncture thereby greatlylessened as the hysteresis loss plays a most important part in causingpuncture.

After the moisture has been removed in this way the insulating materialshould be kept out of contact with the air to prevent it from absorbingmore moisture and air. This may be done by winding it on suitable reelsunder oil and keeping these reels as well, as the conductor immersed ina tank of oil while it is being wound upon the conductor. An additionaladvantage in making an insulated conductor in this manner lies in thefact that all air may be excluded from the finished product. When theinsulating material is heated in single layers all of the air may easilybe removed therefrom, by vacuum treatment, if necessary. and all of thepores therein filled with oil. If the winding operation is also carriedout under oil no air will be entrapped between the layers of insulatingmaterial. As a result the completed cable will be entirely free from airand free from moisture to an extent which it hasbeen impossible toobtain with the previously used methods of manufacture.

In the accompanying drawing Fig. 1 shows an elevation with portions ofthe sheath and insulation broken away of a cable constructed inaccordance with my invention, and Fig. 2 shows a cross-section of thecable. As indicated in the drawing, a conductor 1 is surrounded by aplurality of layers of insulating material 2 from which the moisture hasbeen thoroughly removed by one of the methods which I have described.Each layer is preferably wound with butt joints between the edges of thestrips, the joints between successive layers being broken as indicated.The insulation s covered by the usual lead sheath 3 which 15 drawn overthe insulated conductor as it emerges from the oil bath in which thewinding operation is carried on in such a way that there is noopportunity for air or moisture to be absorbed by the insulation or tobe entrapped between the sheath and the insulation. In passing throughthe oil bath all interstices 4 between the strands of the conductor 1are also filled with oil. The air being thus entirely excluded from thecable there will be no opportunity for the formation of destructurecorona discharges from the conductor when it is employed for highvoltage transmission.

lVhat I claim as new and desire to secure by Letters Patent of theUnited States, is

1. An electrical conductor covered with an insulating material, thedialectric strength of which increases with increase of temperature overa range of temperature from 20 C. to 100 C.

2. Fibrous material for insulating elec trical conductors the puncturevoltage of which increases with increase in temperature over a range oftemperature from 20 C. to 100 C.

3. An electrical conductor covered with a plurality of layers of paperfrom which the moisture has been removed to such an extent that thepuncture voltage increases with increase in temperature over a range oftemperature between 20 C. and 100 C.

4. The process of preparing fibrous insulating material for electricalconductors which consists in treating separate layers of the same incontact with oil at a temperature of at least 100 C. for a long enoughperiod to remove sufiicient moisture to give the material a positivetemperature coefficient in regard to the voltage at which it will bepunctured over a range of temperature from 20 C. and 100 C.

5. Insulating material for electrical con- 10. ductors, thepuncturevoltage of which increases with increase in temperature over arange of temperature from 20 C. to 100 C.

6. An electrical conductor covered with a fibrous insulating material,the dialectric strength of which increases with increase in temperatureover a range of temperature from 20 C. to 100 C.

7. The process of preparing insulating material for electricalconductors which consists in treating it while in contact with aninsulating fluid at a temperature at least as great as the boiling pointof water for a long enough period to remove suflicient moisture to givethe material a positive temperature coefiicient in regard to the voltageat which it will be punctured over a range of temperature from 20 C. to100 C.

8. The process of preparing insulating material for electricalconductors which consists in removing the moisture therefrom to such anextent that it has a positive temperature coeflicient in regard to thevoltage at which it will be punctured over a range of temperature from20 C. to 100 C. and atthe same time preserving its mechanical strength.

9. A cable for electrical purposes comprising an electrical conductor, aplurality of layers of fibrous insulating material the dielectricstrength of which increases with increase in temperature over a range oftemperature from 20 C. to 100 C. surrounding said conductor and aprotective airtight sheath surrounding said insulating material, thespace within said sheath being free from air.

10. The process ofmanufacturing an insulated electrical conductor whichconsists in removing moisture from fibrous insulating material, windingthe insulating material upon a conductor and preventing the insulatingmaterial from coming in contact with the airafter the moisture has beenremoved therefrom and while it is being wound upon the conductor.

11. The process of treating fibrous material which consists in replacinga. portion of its moisture of constitution by an in- 20 sulating fluid.

12. Fibrous material for insulating electrical conductors having aportion of its moisture of constitution replaced by an insulating fluid.

13. An electrical conductor covered with an insulating material in whicha portion of the moisture of constitution has been replaced by aninsulating fluid.

14. An electrical conductor covered with a plurality of layers of paperin which a portion of the moisture of constitution has been replaced byan insulating fluid.

15. The process of removing moisture from insulatingmaterial which.consists in impregnating it with an insulating fluid heating thematerial and then causing dry heated air at 'a pressure lower thanatmospheric to pass over the material.

In witness whereof, I have hereunto set my hand this 8th day of January1918.

ELMER E. F. CREIGHTON.

- Certificate of Correction;

, It is hereby certified that in Letters Patent No. 1,542,863. grantedJune 23, 1925. upon the application of Elmer E. F. Creighton, ofSchenectady, New York, for an improvement in Word destructure readdestructive;

Insulated Electrical Conductors, specification requiring correction asfollows: Page lines 75 and 105, claims land 6, tor the word errorsappear in the printed 2, line 69. for the misspelled dialectric readdielectric, and line 99, claim 4, for the word and read to; and

that the said Letters Patent should same may conform to the be read withthese c'orrectionstherein that the record of the case in the PatentOtlice. Signed and sealed this 28th day of July, A. D. 1925.

KARL FENNING, Acting Commissioner of Patents.

